They knock you out for it?
In most cases, at least now. I had one when I was in my 20s and they didn't put me out for it. That was insanely uncomfortable. It turned me off from even having an appointment with a gastroenterologist for a long time.
I finally gave in a few years ago. I was 48. Of course, he had me schedule a colonoscopy on that first visit. I had been through a corneal transplant, three surgical procedures on my foot, and arthroscopic surgery for a torn meniscus in my left knee, it was nothing new for me to be put under. I was very relieved when the doctor told me that I would be out for my colonoscopy.
Prep does suck, but it's not that awful. It's a small price to pay when it comes to your health. The gown is awkward, but again, that was nothing new for me. Nobody mentioned positioning, so maybe it was different for some of you. They told me to lay on my side with my knees up in front of me. Forget the gown, the real comical image is me on the table with my ass hanging out. Even better, there was some delay before they were ready, so I just had to lay there like that with the doctor, the anesthesiologist and a nurse doing what they needed to do to set up. Then the start the meds and I'm out. I tend to have a little bit tougher of a time waking up after anesthesia, so I remember meeting with the doctor and asking him the same question five or six times.
I do believe that in some cases they might use twilight sedation rather than general anesthesia. With twilight, it is more like being asleep, so there is more of a chance of the patient moving. The doctors got a reminder of that during my first foot surgery. They gave me a nerve block so I couldn't feel anything below my knee for 32 hours, so they thought they could go with twilight. Unfortunately, while my foot was cut open, I kicked out and brushed my foot against a surgical curtain. That must've been fun. Then I woke up with a half hour to go in the procedure. I couldn't feel anything and I was loopy from the meds, so I kept asking if I could it up and watch.

Needless to say, when they had to go back in for a second procedure, they went for the general anesthesia.